It is recognized in the parison molding art that sufficient cooling of parisons is indispensable if the parisons are to maintain their shape upon withdrawing or pulling out of the core rod deployed at the injection molding station and during later handling operations. A high rate of production of parisons is important in commercial operation, and the rate at which the injection molding cycle can produce the parisons has been limited by the time taken to cool the parisons sufficiently to allow post molding handling without damage to the parison.
Typically, the hollow plastic parisons produced in the injection molding cycle have relatively thick walls and are molded with PET resin at relatively high temperatures. Consequently, prior to removal from the injection mold, the hollow parison must be cooled sufficiently to prevent deformation during removal and during transit and must be cooled to glass transition temperatures so as to reduce crystalization and allow orientation. However, the thick parison wall tends to trap heat. Cooling the parison in the injection mold by means of its contact with the cool mold and core rod surfaces quickly chills the inner and outer skin, but mold cooling will remove the heat trapped within the parison wall only by keeping the parison in the injection mold for an extended period of time. Naturally, this is economically prohibitive because high parison production and ultimately bottle protection is not feasible unless cooling of the parison in the injection mold is held at a minimum.
Moreover, there are many applications for parisons possessing multiple layers of resin to form multiple layered packages, bottles or other hollow objects. For example, one or more of the layers could add barrier properties to parisons of polyethylene terapthalate (PET) and the ultimately molded bottles. The inner layer could be virgin PET and the center or outer layer could be recycled post consumer usage material with a third layer being a barrier-type resin. Even with the formation of multiple resin layered parisons, the rate at which parisons are produced is limited by the time taken to cool the parisons sufficiently to allow handling after each layer is injection molded without damage to the parison.